Polytetramethylene ether glycols, which are useful soft segments for applications such as polyurethane elastomers and thermoplastic polyesters, are produced by polymerizing tetrahydrofuran in the presence of a cationic initiator. Commercially important cationic initiators include strong, soluble acids such as fuming sulfuric acid and fluorosulfonic acid. Polytetrahydrofuran diols produced using the soluble acid initiators have a sufficiently narrow molecular weight distribution (M.sub.w /M.sub.n less than about 3) for many applications. Unfortunately, these processes generate large amounts of acidic wastes that require costly treatment and disposal. In addition, product consistency is difficult to achieve because the polymerizations are highly exothermic, and satisfactory product consistency requires careful control of reaction temperature.
Many of the problems of the soluble acid catalysts are avoided by using a "bleaching earth" catalyst (see, for example, U.S. Pat. Nos. 3,433,829, 4,189,566, and 4,243,799). The bleaching earths known in the art include naturally occurring aluminum hydrosilicates and aluminum/magnesium hydrosilicates of the montmorillonite type. The clays are normally activated by acid washing. A carboxylic acid anhydride is used as an activator, and the resulting polytetramethylene ether polymer has ester end groups. The ester end groups can be converted to hydroxyl end groups by base-catalyzed transesterification with an alcohol (see U.S. Pat. No. 4,230,892) or by catalytic hydrogenation.
A key disadvantage of the bleaching earth catalysts is that the polymers produced have higher polydispersities (M.sub.w /M.sub.n) than desirable, typically 3-4 at molecular weights of about 400 to 3000. It is well known in the art that the molecular weight distribution (MWD) of the poly(THF) impacts the properties of the polyurethanes or polyesters made therefrom. In general, mechanical properties of finished products are superior when poly(THF) having a relatively narrow molecular weight distribution is used (see U.S. Pat. No. 4,933,503, column 2).
There are two general approaches to obtaining poly(THF) having a relatively narrow MWD. In one approach, poly(THF) having a broad MWD is prepared, and the product is post-treated either by distillation to separate low molecular weight oligomers, selective depolymerization (see, for example, U.S. Pat. No 4,363,924), selective solvent extraction with water/alcohol/hydrocarbon systems (see U.S. Pat. No. 4,762,951), or a combination of these techniques (see U.S. Pat. No. 4,933,503). All of these post-polymerization techniques are expensive, labor-intensive, and time consuming. The object of the second general approach is to eliminate the need for post-treatment by preparing poly(THF) having a narrow MWD. In one method, a low concentration of an alkylene oxide must be maintained throughout the THF polymerization (U.S. Pat. No. 4,728,722). In another method, the mole ratio of the reactants and reaction temperature must be carefully controlled (U.S. Pat. No. 4,510,333).
A solid acid catalyst with the advantages of the bleaching earth catalysts, yet one that inherently gives polymers having a narrower molecular weight distribution than what is possible with the bleaching earths (and therefore overcomes the need for post-treatment), is needed.